The present invention relates to suspension polymerized alkyl vinyl copolymer with styrene and to its rubber modified impact; and more particularly, to a copolymer of monomeric higher alkyl vinyl with styrene grafted in suspension polymerization to a pure preformed rubber; and to the method of formation of these products by suspension polymerization.
According to the present invention, in a first aspect, it is found that styrene having its polymeric backbone modified as a copolymer with a minor quantity, less than about 15% of a monomeric alkyl vinyl compound has a built-in lubricating effect, superior in easy-flow molding to more perfect castings at lower molding pressures. The copolymer has a better stability of its lubricating properties in storage being resistant to variations over long storage periods, a common fault of crystal polystyrene particles which have been blended with extraneously added lubricants for molding. It is found moreover that this copolymer of such monomeric higher alkyl vinyl compound will expand with a foaming agent to a foam with smaller cells, a substantial advantage over many foamed products formed from foamable polystyrene.
In a second aspect, the invention includes the copolymer of a higher alkyl vinyl and styrene formed by suspension polymerization in the presence of preformed rubber particles, whereby the copolymer graft polymerizes to the rubber, forming superior high impact copolymer beads, both easily moldable, with good flow characteristics and of superior impact resistance.
The structure of applicant's copolymers are essentially the suspension polymerization copolymer of monomeric styrene with a minor quantity such as about 0.25 to 15%, preferably 0.5 to 6% by weight of a higher alkyl vinyl compound based on the weight of the copolymer without rubber. Where the final polymer is the ternary polymer to include preformed rubber, the polymer composition will consist of from 0.25 to 10% of such higher alkyl vinyl, 2.0 to 10% of preformed rubber, with the remainder being substantially styrene, copolymerized in suspension polymerization with the higher alkyl vinyl, and graft polymerized both as polymer and copolymer to the preformed rubber.
There are minor quantities of other components usually added in the suspension copolymerization to form the polymeric product as well as to improve its molding properties, namely small quantities of catalysts such as free radical catalysts typically an organic peroxy compound of the formula ##EQU1## in which R is hydrocarbon, such as benzoyl peroxide, may be used. Suspension agents, typically fine particles of alkali earth metal phosphates such as calcium phosphate; surface-active agents such as higher alkyl sulfates typically dodecyl benzene sulfonate; polymer chain modifiers, typically alkyl mercaptans; and extra mineral lubricating oil may be present, generally in small quantity ranging from about 0.01% up to about 5%. Such additives serve to control the suspension polymerization and will generally be used in quantity of less than 1%, and where mineral lubricating oil is used, it is optional and would be present in quantity of about 1 to 5%.
The higher alkyl vinyl is a monomeric 1-olefin having at least 14 carbon atoms ranging up to about 60 carbon atoms and preferably will be in the range of 16 to 26 carbon atoms. The styrene is the common polymer base substance normally used herein but may be replaced with other vinyl aromatics, typically vinyl toluene, vinyl xylene, vinyl ethyl benzene and alphamethylstyrene to form similar products.
In forming the product, it is usual first to dissolve the rubber particles in polystyrene when an impact is being formed, mix the styrene monomer and alkyl vinyl monomer together with preformed rubber, add the small quantity of additives, catalyst, lubricating agents, etc., generally without the suspending agents, and then heat preliminarily with agitation at a temperature below about 90.degree.C for 1-4 hours to initiate the polymerization as a mass prepolymerization reaction. That mass-polymer generally will be agitated until the mass becomes more viscous and inversion of the rubber occurs. After initial prepolymerization is completed, the prepolymer is suspended in water to which calcium phosphate particles have been added, the viscous liquid and water varying from 35 to 65% for either, continuing the agitation of the suspension in water with high speed mixing while slowly raising the temperature to the range of 95.degree.-125.degree.C under the autogenous pressure developed in a closed container for a total period of 7-24 hours of polymerization time. At that point the polymerization will have been completed to convert the polymer to small bead-like solids, the particles ranging in size from about 30-100 mesh U.S. Standard screen. The polymerization and heating are then discontinued, the settled particles are separated by decantation and are washed with dilute acid such as 10% HCl, rinsed in water to neutrality and dried. Such process of suspension with mass prepolymerization is generally shown in U.S. Pat. No. 3,188,364. While it is preferred to prepolymerize before suspension polymerization, the entire reaction may be carried out as normal suspension polymerization such as shown in U.S. Pat. Nos. to Dyer, et al 3,047,534, Meek, et al. 3,100,763 and White 3,068,192, without preliminary polymerization of the mixed monomers. These patents are here incorporated by reference.
While various commercial rubbers will improve the impact strength of the copolymer hereof, it is preferred to use a relatively pure polybutadiene such as Poly SAR, a product of Polymer Corporation in which the rubber is a polybutadiene having a 98% cis content, the 1-2 vinyl structure therein being less than 2% and is almost 100% soluble as a gel in styrene monomer. Other preferred rubbers are known as DIENE sold by the Firestone Tire & Rubber Company as DIENE 55 having a 38% approximate cis content with a narrow molecular weight range and DIENE 35 having an approximate 32% cis content and slightly lower MW range.
With respect to the reduction in cell size, foamed crystal styrene normally has cells ranging from 50-100 m.mu., but the copolymer hereof will be reduced to the approximate range of 20-50 m.mu. by the styrene copolymerization with the higher alkyl vinyl compound.
As to impact resistance, as well known in the art this varies somewhat with the quantity of rubber used, but it usually takes from 10-15% of graft polymerized rubber of the DIENE type to achieve an izod impact strength of 4-5 ft. lbs./in. in the product. The presence of 0.5 to 10% of higher alkyl vinyl copolymerized with the styrene will achieve an impact strength of 4-6 ft. lbs./in. with a moderate 6% rubber content.
Again, with respect to the inherent lubrication effect of the copolymer hereof, that is containing 0.5 to 10% of higher alkyl vinyl, the flow index of the copolymer is more than doubled. This copolymer may be polymerized to a higher molecular weight resulting from the copolymerized presence of the higher alkyl vinyl in the copolymer and this imparts a lubrication effect built into its molecular structure as well as higher shock resistance properties. It overcomes the problem, being self-lubricating during extrusion and does not lose lubricant as does an externally lubricated polymer in which the lubricant, such as mineral oil, is added in an extraneous mixture, and much of which may substantially be lost during storage and extrusion.
The modified higher alkyl vinyl impact, however, while having a correspondingly higher molecular weight also has a higher stress-crack resistance, thereby being a much tougher polymer.